Emperor Constantine’s Nicene council is usually pointed to as the source for the doctrine of the Trinity, with Athansius as the one who INFLUENCED him. Yet the Trinity was present in the church LONG before Constantine.
The term Trinitas was popularized by Tertullian almost 100 years before the Nicene council in his debate against Praxeas.
However, he was not the first to use the term, a man Theophilus Bishop of Antioch in 160 was the first to use the term (that we have in writing), many years before in his epistle to Autolycus The 2nd,xv.
We can assume it was used prior to Theophilus and was held as a common church belief with the many quotes that are left to us in history by the early church pastors. Athenagoras representing the whole Churches belief wrote, that, “they hold the Father to be God, and the Son God, and the Holy Spirit, and declare their union and their distinction in order.”(A plea for the Christians.10.3)
The term was used to simply describe the three that simultaneously exist as the one God. A man named Praxeas promoted what is called Monarchianism, which held a strict form of monotheistic progression. That the Father became the Son, and the Son became the Spirit. This is what is called modalism in it’s simplest form, What is better termed Oneness today. Despite the accusation’s of the Church inventing and promoting the Trinity. We find the Church in Rome and elsewhere falling prey to numerous heresies that they tried to keep out.
So, the Trinity did not depend on any council as it was used by Tertullian and others long before a council was called on doctrinal teaching.
however, according to Tony Lane in his The Lion Concise Book of Christian Thought (Lion, Oxford, England, 1992 edition) :
Tertullian (160?-230?), who has been called the father of Latin theology, coined the terms later used to define the Incarnation and the Trinity. He wrote that God is one substance in three persons — Father, Son, and Holy Spirit — but he did not portray them as equal in precedence or power or esteem. Judged theologically, he was almost a Jew. [Lane]
Origen (185-254) has been called the greatest and most influential Christian teacher between Paul and Augustine. He described a three-level divine hierarchy in which the Father is greater than the Son, and the Son is greater than the Holy Spirit. He was not certain whether the Holy Spirit should be considered a person or a principle. At this time, many Christians believed as Origen did. [Lane]
Paul of Samosata, Bishop of Antioch 260-272, emphasized the sovereignty of God and the humanity of Christ. He said Jesus was a sinless man, uniquely united with God in will and purpose. By his perfect obedience despite his temptations, struggles and suffering, Jesus overcame the sin of Adam and grew in intimacy with God. This doctrine of “Low Christology” was condemned by a local synod. However, the same synod also rejected use of the term homo-ousios (of the same substance) in reference to the relationship of God and Christ. [Lane]
Thus there was not the trinity as postulated by Athanasius before he went on his nearly 40 year crusade to make his concept of the Trinity as the sole doctrine of the Christian Church.